1. Field of the Invention
The present invention relates to a circuit that generates a clock signal. In addition, the present invention relates to a semiconductor device provided with a circuit that generates such a clock signal.
2. Description of the Related Art
In recent years, a semiconductor device in which an ultra-small IC chip and an antenna for wireless communication are combined with each other (also referred to as an RFID tag, a wireless tag, an ID tag, or an RF tag) has been attracting attention. The semiconductor device can transmit and receive data without contact, for example, it can write data or read data, by transmission and reception of a communication signal by use of a wireless communication device (which can communicate wirelessly, for example, a reader/writer, a cellular phone, a personal computer, or the like).
As an application field of a semiconductor device which transmits and receives data by a wireless signal, product management in the distribution industry is given as an example. Product management using a bar code and the like is the mainstream at present; however, since bar codes are read optically, data cannot be read in some cases when an interrupting object exists. On the other hand, when data is transmitted and received without contact by use of a wireless communication device, data of the semiconductor device is read wirelessly; thus, data can be read even when an interrupting object exists, if the interrupting object allows a wireless communication signal to pass through. Accordingly, improvement in efficiency, cost reduction, and the like of product management are expected. Further, a wide range of applications including boarding passes, airplane tickets, and automatic payment of fares are expected. A system such that people and objects are identified and managed by a small semiconductor device which transmits and receives data by wireless communication is called radio frequency identification (RFID) and has attracted attention as fundamental technology of the IT society.
RFID technology is one of the central technologies of the ubiquitous computing era. However, RFID tags which form the basis of RFID technology have the following problems: for example, physical flexibility, high secrecy, and economical efficiency. To meet these requests, there are a semiconductor device (hereinafter referred to as an HF wireless communication CPU) which is formed over a flexible substrate or a glass substrate and uses a wireless signal of an HF band (13.56 MHz), and the like (see Reference 1: H. Dembo et al. “RFCPUs on Glass and Plastic Substrates Fabricated by TFT Transfer Technology,” IEDM Tech. Dig. Papers, pp. 1067-1069, 2005).
Furthermore, in order to construct an RFID system which is more functional than the above HF wireless communication CPU, a high level of functionality is continuously required for RFID tags; for example, communication distance is extended, power consumption is lowered, an encoding function is enhanced (resistance to side-channel attacks), and the like. For example, as the resistance to side-channel attacks, a Transformed Masking Method (see Reference 2: M. Akkar and C. Giraud, “An implementation of DES and AES, secure against some attacks,” Proceedings of CHES 2001, LNCS 2162, pp. 309-318, 2001) and the like can be given.